The present invention relates to a bias circuit and semiconductor device, and more particularly, this invention relates to a bias circuit which applies an optimal bias in response to a “variation” in thresholds of transistors in various kinds of semiconductor device, such as a high frequency power amplifier using MESFET (metal semiconductor field effect transistor) or HEMT (high electron mobility transistor), and a semiconductor device using the same.
A bias circuit which supplies a bias to a transistor has an important role in many cases in various kinds of semiconductor device, such as a high frequency amplifier, a mixer, a modulator and an oscillator which comprise transistors, such as MESFET and HEMT.
Hereafter, the high frequency amplifier which comprises MESFET will be mentioned as an example, and will be explained as an example of such a semiconductor device.
FIG. 7 is a schematic diagram showing the principal part of the high frequency amplifier which was attempted by the Inventors of the present invention in the course of attaining this invention. This amplifier is a power amplifier using MESFET (FET) which consists of gallium arsenide (GaAs), and for example, can be used as a front end amplifier for a cellular phone.
The amplifier illustrated in FIG. 7 has the amplification circuit PA and the bias circuit BC. The resistance R103 built in the bias circuit BC has a role to isolate a bias circuit from an amplification circuit at high-frequency.
This bias circuit BC consists of a voltage divider which includes resistances R101 and R102.
A voltage supplied to node n1 is divided at a constant ratio and is supplied to the input terminal n2 of an amplification circuit. In the amplification circuit PA, RF input is given to n3, and the drain power supply of FET is connected to n4. And RF output is obtained at n5.
However, in the case of the amplifier illustrated in FIG. 7, since the gate bias voltage outputted from the bias circuit BC is constant, when the threshold of FET of the amplification circuit PA varies, there is a problem that the drain current of the FET also varies according to the variation of the threshold.
In many cases, the consumption of the battery by a power amplifier may be a large part of a total consumption of the battery in mobile communications apparatus etc. Therefore, in order to lengthen the operation time of a battery, it is necessary to set a maximum limit in the drain current of a FET in a power amplifier.
However, since characteristic degradation including the decrease of gain and increase of distortion will arise if drain current is set low, it is also necessary to set a minimum limit in the drain current. Since drain current varies by a threshold in the case of the amplifier illustrated in FIG. 7, it is also necessary to set allowable maximum and minimum limits in the “variation range” of the threshold of FET.
However, since manufacturing process conditions vary for every wafer, “variation” arises also in a threshold in various kinds of transistors, such as GaAs MESFET. Besides, a “variation” in a threshold may also arises in a wafer, since process conditions may not be same throughout a wafer.
Such a “variation” in a threshold has become the cause of reducing a yield of production of power amplifiers.
The same situation is also seen in other application fields. That is, it is very important to suppress the “variation” in the operating characteristic resulting from the “variation” in the threshold to the minimum, in various kinds of semiconductor device using a transistor.